Semiconductor optical amplifier

ABSTRACT

The invention relates to a semiconductor optical amplifier including a buried guide active structure ( 12 ), characterized in that the guide active structure ( 12 ) is subjected to an external stress to render the gain of said amplifier insensitive to the polarization of the light to be amplified, said external stress coming from a force induced by a deposit of a material ( 50 ) against a ridge ( 15 ) surrounding said guide active structure ( 12 ).

[0001] The present invention relates to amplifying optical signals. Itfinds a typical application in fiber optic telecommunication networks.The signals transmitted by fiber optic telecommunication networksconsist of pulses carrying information to be transmitted in binary form.The pulses must be amplified to compensate power losses that they sufferduring their propagation in said networks. Semiconductor amplifiersconstitute a compact means of obtaining such amplification and can beintegrated. However, unless specific measures are implemented to preventit, their gain is sensitive to the state of polarization of the lightthat they receive, as indicated more simply hereinafter by referring tothe polarization-sensitivity of an amplifier.

[0002] The invention finds a particular application when it is necessaryto eliminate or at least limit polarization-sensitivity, which can beexpressed by the following equation: ΔG=G_(TE)−G_(TM). The aim is toachieve the condition |ΔG|<1 dB.

[0003] The situation in which the sensitivity must be limited oreliminated is frequently encountered and arises when the distancetraveled by the optical pulses to be amplified is such that the state ofpolarization of the pulses has been significantly and randomly affectedduring their propagation and it is preferable for the amplified pulsesto have one or more predetermined power levels.

[0004] More generally, the invention finds an application whenever anoptical amplifier must have no polarization-sensitivity or a lowpolarization-sensitivity.

[0005] The invention applies more specifically to buried ridge structure(BRS) amplifiers.

[0006] A buried ridge structure semiconductor optical amplifier (seeFIG. 1) includes a wafer 2 made up of layers of semiconductor materialshaving respective refractive indices and forming a common crystallattice. In the absence of mechanical stresses, the lattices formed bythese materials have respective characteristic dimensions constitutingrespective lattice parameters of the materials. The layers are insuccession in a vertical direction DV forming a right-angle trihedron,defined with respect to the wafer 2, with two horizontal directionsconstituting a longitudinal direction DL and a transverse direction DT.The layers form an upward succession in the vertical direction DV from abottom face 4 to a top face 6. The wafer 2 includes at least thefollowing layers or groups of layers or parts of layers:

[0007] A substrate 8 consisting mainly of a semiconductor basic materialhaving a first type of conductivity. This substrate is sufficientlythick to impose the dimensions of the lattice of the basic material onall of the crystal lattice of the wafer 2.

[0008] An active layer 10 including an active material adapted toamplify light by stimulated recombination of charge carriers of bothtypes injected into the material.

[0009] A buried guide active structure 12 formed in the active layer 10and having a higher refractive index than the surrounding materials 14,16. The active structure 12 extends in the longitudinal direction DL toguide light in that direction and has a transverse width 1 and avertical thickness e.

[0010] Finally, a top confinement layer 18 consisting of a materialhaving a second type of conductivity which is the opposite of the firsttype.

[0011] The amplifier further includes a bottom electrode 20 and a topelectrode 22 respectively formed on the bottom face 4 and the top face 6of the wafer 2 to enable an electrical current to flow between saidfaces for injecting said charge carriers of both types into the activematerial 10.

[0012] The basic material of prior art semiconductor optical amplifiersare III-V materials, typically indium phosphide and gallium arsenide.The active material is typically a ternary or quaternary materialcontaining the same chemical elements. It is generally required for thewidth 1 of the guide active structure 12 which guides the light to beclose to one micrometer, to facilitate etching it and most importantlyto facilitate integrating the amplifier with other optical components onthe same semiconductor wafer. To ensure monomode propagation of light,typically at a wavelength of 1,310 nm or 1,550 nm, the thickness e mustthen be very much less than the width 1. If no special measures areapplied to prevent it, this rectangular shape of the section of theguide active structure 12 causes the polarization-sensitivity previouslymentioned.

[0013] In BRS amplifiers, the active material 10 constituting the activestructure 12 guiding the light is surrounded on all sides by a binarysemiconductor material. This material has the advantage of conductingheat well, but its refractive index is only slightly lower than that ofthe active material. Consider further the situation in which the activematerial is homogeneous, in which case it is referred to as a bulkmaterial. As a general rule, the section of the buried guide activestructure 12 is strongly rectangular. Given the small index differencebetween the guide structure 12 and the surrounding binary material, theconfinement of a horizontally polarized wave is greater than that of avertically polarized wave, the difference between the two confinementfactors increasing as the ratio of the width 1 to the thickness e of theguide structure 12 increases. The confinement referred to above inconnection with a wave is in a transverse plane. It is equal to theratio of the power of the wave in the area occupied by the guidestructure to the total power of the wave. The confinement factor isdefined for each polarization and for each wavelength by the shape anddimensions of the section of the guide active structure 12 and by therefractive indices of the material of the structure 12 and thesurrounding materials 14, 16, 8 and 18. In the case of a rectangularsection, it can be considered to be the product of a directionalconfinement factor in the horizontal direction by a directionalconfinement factor in the vertical direction, each of the twodirectional confinement factors depending on the polarization. Giventhat the phenomenon of amplification of the wave by recombination ofcarriers and stimulated emission occurs only in the active material,i.e. in the structure 12, the gain of the amplifier for a wave increasesas the confinement of the wave increases. As a result of this, if thematerial of the guide structure 12 were a homogeneous material, and alsoisotropic, and therefore insensitive to polarization, the gain of theamplifier would be greater for horizontally polarized waves than forvertically polarized waves.

[0014] Considerable research has been conducted into making theseamplifiers insensitive to the polarization of the light to be amplified.

[0015] In particular, U.S. Pat. No. 5,982,531 proposes an amplifier ofthis kind that is rendered insensitive to the polarization of the light.The amplifier is characterized in that its active material 10 issubjected to a sufficient tension stress to render its gain insensitiveto the polarization of the light to be amplified. This stress generallyresults from a lattice mismatch between the active material 10 and thebasic material of the substrate 8. The horizontal confinement factor istypically equal to the product of the vertical confinement factor by aconfinement dissymmetry coefficient.

[0016] The present invention is based on the observation that, even inthe presence of a high confinement dissymmetry coefficient resulting,for example, from the fact that the guide structure 12 consists of aridge of strongly rectangular section, the tension stress to be appliedto a homogeneous active material forming said structure to obtaininsensitivity to polarization is sufficiently low for the thickness ofthe structure to remain less than the corresponding critical thicknessrelating to dislocations.

[0017] The above kind of amplifier has a low sensitivity topolarization. Its main parameters are:

[0018] the wavelength of the amplifying active layer: λ=1.57 μm,

[0019] the active material: In_(1−x)Ga_(x)P_(1−y)As_(y),

[0020] the tension stress of the active layer: Δa/a =−0.015,

[0021] the thickness of the guide structure: e=0.2 μm, and

[0022] the width of the guide structure: 1=1 μm.

[0023] This kind of structure has drawbacks, however. It has beenestablished experimentally and theoretically that the polarizationdepends strongly on the control of the thickness of the active layer andthe stresses to which it is subjected. For example, modifying thisstress (Δa/a) from −0.015 to −0.014 or −0.016 induces a gain shift ΔG of0.8 dB toward a respective sensitivity of the TE mode or the TM mode.Similarly, a slight modification of the thickness of the guide activestructure (of a few percent) directly induces a gain offset ΔG of theamplifier. Accordingly the sensitivity of the amplifier to thepolarization of the light depends on its structure and cannot becontrolled easily.

[0024] The object of the present invention is to remove the drawbacks ofthe technology proposed by the previously cited U.S. Pat. No. 5,982,531.

[0025] The invention begins from the theoretical and experimentalfinding that the sensitivity of a semiconductor optical amplifier to thepolarization of the light to be amplified depends not only on thegeometry of the guide active structure and internal stresses to which itmay be subjected (such as a lattice mismatch), but also on externalstresses applied to the guide active structure, for example duringimplementation or use of the optical component.

[0026] To this end, the invention proposes to apply to the amplifiercomponent an external stress induced in the vicinity of the guide activestructure, alone or in combination with an internal tension stress,depending on the epitaxial structure of the active layer, as developedin the prior art. The external stress can have its parameters set toobtain an overall stress on the guide active structure that renders thegain of the optical amplifier insensitive to the polarization of thelight to be amplified.

[0027] The invention relates more particularly to a semiconductoroptical amplifier including a buried guide active structure,characterized in that the guide active structure is subjected to anexternal stress to render the gain of said amplifier insensitive to thepolarization of the light to be amplified, said external stress comingfrom a force induced by a deposit of a material against a ridgesurrounding said guide active structure.

[0028] According to one feature, the ridge surrounding the guide activestructure has a depth from 1 μm to 4 μm.

[0029] According to another feature, the ridge surrounding the guideactive structure has a width from 4 μm to 6 μm.

[0030] According to another feature, the material deposited against theridge surrounding the active structure is an oxide.

[0031] In one embodiment, the material deposited is SiO₂ for an externalcompression stress.

[0032] In another embodiment, the material deposited is Si₃N₄ for anexternal tension stress.

[0033] According to one feature, the thickness of the material depositedagainst the ridge surrounding the active structure is from 0.1 μm to 0.5μm.

[0034] In one application, the guide active structure is subjected to aninternal stress due to a lattice mismatch of the semiconductor material,the external stress being a stress additional to this internal stress.

[0035] The invention also relates to a method of manufacturing asemiconductor optical amplifier, characterized in that it includes thefollowing steps:

[0036] producing a semiconductor optical amplifier with a buried guideactive structure,

[0037] etching a ridge surrounding said guide active structure,

[0038] depositing an oxide on the contours of the etched ridge, and

[0039] forming an electrode.

[0040] The optical amplifier according to the invention achieves asensitivity to the polarization of the light less than or equal to 1 dB.

[0041] The ridge surrounding the guide active structure can be etched atthe end of the process of fabricating the optical amplifier, usingtechniques that are known in the art and easy to use.

[0042] Other features and advantages of the present invention willbecome apparent in the course of the following description, which isgiven by way of non-limiting illustrative example and with reference tothe drawings, in which:

[0043]FIG. 1, already described, is a diagram of a prior art buriedridge structure semiconductor optical amplifier.

[0044]FIG. 2 is a diagram of a semiconductor optical amplifier inaccordance with the invention subjected to an external stress.

[0045]FIGS. 3a to 3 d show diagrammatically steps in the production ofan optical amplifier according to the invention.

[0046] The following description refers to FIG. 2 and to FIGS. 3a to 3d.

[0047] An optical amplifier according to the invention has a structureanalogous to that described in connection with the prior art withreference to FIG. 1. The same reference numbers are used to designatethe same components. The method of fabricating this kind of amplifiertherefore includes the same steps, producing an amplifier with a buriedguide active structure 12.

[0048] In a first application, the guide active structure 12 can besubjected to an internal stress due to a lattice mismatch of thesemiconductor material, as described with reference to U.S. Pat. No.5,982,531. The invention then consists in completing or compensatingthat internal stress by applying an external stress to the guide activestructure 12 so that it is subjected to an overall stress that rendersthe gain of the amplifier insensitive to the polarization of the lightto be amplified.

[0049] In another application, no internal stress is applied to theguide active structure 12 during epitaxial growth of the active layer10, and the invention then consists of applying an external stress tothe guide active structure 12 so that it is subjected to an overallstress that renders the gain of the amplifier insensitive to thepolarization of the light to be amplified.

[0050] In either case, the external stress comes from a force induced bydepositing a material 50 against a ridge 15 surrounding the guide activestructure 12.

[0051] The ridge 15 surrounding the guide active structure 12 is etchedby any technique known in the art, for example by dry etching through amask 17 deposited on the confinement layer 18 of the amplifier, forexample an SiO₂ mask.

[0052] The ridge 15 preferably has a depth from 1 μm to 4 μm and a widthfrom 4 μm to 6 μm.

[0053] As a general rule, the buried guide active structure 12 has awidth of approximately 2 μm and is at a depth from the top face 6 of thecomponent from 2 μm to 4 μm (for a component approximately 100 μm thickincluding the substrate 8). The ridge 15 therefore surrounds the guideactive structure 12 and the forces exerted on the ridge 15 aretransmitted to the active structure 12 to influence the overall gain ofthe amplifier and its sensitivity to the polarization of the light to beamplified.

[0054] The material 50 deposited against the ridge 15 surrounding theactive structure 12 is preferably an oxide, for example SiO₂ for anexternal compression stress (dashed line in FIG. 3c) or Si₃N₄ for anexternal tension stress (solid line in FIG. 3c). The thickness of thematerial 50 is advantageously from 0.1 μm to 0.5 μm.

[0055] The parameters for adjusting the external stress on the ridge 15surrounding the guide active structure 12 of the amplifier areessentially the width and the depth of etching of the ridge 15, thenature of the material 50 deposited, and its thickness.

[0056] An electrode 22 is then formed on the top face 6 of the opticalcomponent, for example by forming an opening in the oxide deposit 50 ontop of the active structure 12 to deposit metallization therein.

[0057] The metallization can be deposited on the opening of theelectrode 22 and on top of the external stress material 50.

1. A semiconductor optical amplifier including a buried guide activestructure (12), characterized in that the guide active structure (12) issubjected to an external stress to render the gain of said amplifierinsensitive to the polarization of the light to be amplified, saidexternal stress coming from a force induced by a deposit of a material(50) against a ridge (15) surrounding said guide active structure (12).2. An optical amplifier according to claim 1, characterized in that theridge (15) surrounding the guide active structure (12) has a depth from1 μm to 4 μm.
 3. An optical amplifier according to any of claims 1 to 2,characterized in that the ridge (15) surrounding the guide activestructure (12) has a width from 4 μm to 6 μm.
 4. An optical amplifieraccording to any of claims 1 to 3, characterized in that the material(50) deposited against the ridge (15) surrounding the active structure(12) is an oxide.
 5. An optical amplifier according to claim 4,characterized in that the material (50) deposited is SiO₂ for anexternal compression stress.
 6. An optical amplifier according to claim4, characterized in that the material (50) deposited is Si₃N₄ for anexternal tension stress.
 7. An optical amplifier according to anypreceding claim, characterized in that the thickness of the material(50) deposited against the ridge (15) surrounding the active structure(12) is from 0.1 μm to 0.5 μm.
 8. An optical amplifier according to anypreceding claim, characterized in that the guide active structure (12)is subjected to an internal stress due to a lattice mismatch of thesemiconductor material, the external stress being a stress additional tothis internal stress.
 9. A method of manufacturing a semiconductoroptical amplifier according to any of claims 1 to 8, characterized inthat it includes the following steps: producing a semiconductor opticalamplifier with a buried guide active structure (12), etching a ridge(15) surrounding said guide active structure (12), depositing an oxide(50) on the contours of the etched ridge (15), and forming an electrode(22).